Chemical-mechanical or chem-mech polishing (CMP) has recently become a key technology driver in the semiconductor industry. Chem-mech polishing is the preferred technique to achieve global wafer planarization for submicron advanced semiconductor integrated circuits at the personalization level. As a result of its strong anisotropic removal property, it is extensively used to date for the formation of the conductive interconnection scheme at the surface of semiconductor wafers (hereinbelow referred to as "wafers" for brevity). Forming said conductive interconnection scheme generally requires a refractory metal, such as tungsten (W), which is preferred because of its high conductivity and low corrodability.
Two types of tungsten structures are formed using the chem-mech polishing technique at this stage of the wafer planarization process. They must be distinguished depending on they are horizontal such as lands (for dual damascene and local interconnects) or vertical such as studs (for inter-level contact in the via-holes). However, the same sequence of processing steps can be used to form either type of structure.
For instance, using standard photolithographic and insulator plasma etch techniques, a recessed pattern (lines or openings) is first defined in a planarized dielectric layer. Then, a dual adhesion layer of titanium/titanium nitride forming a liner is deposited onto the wafer by sputtering. Next, a layer of tungsten is blanket deposited onto the wafer surface by Chemical Vapor Deposition (CVD) to fill the recessed pattern.
Finally, the wafer is chem-mech polished with an adequate slurry to remove the tungsten and titanium-titanium nitride materials in excess and to produce a planar semiconductor structure wherein the tungsten and the dielectric surfaces are coplanar. For instance, ferric nitrate/alumina slurries are commonly used to date in the semiconductor industry for that purpose. Alumina is the most commonly used abrasive for polishing tungsten because it is closer in hardness than any other material. Adhesion of particulate contaminants occurs as a result of the electrostatic attractive forces between particles (alumina, metals, . . . ) and some of the wafer constituents (w, SiO2, . . . ) during the polishing step properly said. The zeta potential is the parameter which usually provides a measure of these electrostatic forces between charged particles.
The CMP polishing step is generally continued by the so-called "touch-up" step with a specific slurry. Its main role is to terminate the polishing action of the previous CMP step, it also strips away some metallic residues left at the wafer surface that could subsequently reveal detrimental in the successive processing steps (e.g. to cause electrical shorts between conductive lands). However, this step has a limited cleaning effect. During this step, a very thin (50 nm) superficial film of the dielectric layer is etched, typically with an ammonium persulfate or an alkaline silica based slurry. The "touch-up" step is sometimes considered as optional, but in reality it is quite recommended.
At this stage of the tungsten chem-mech polishing process, an efficient cleaning procedure must necessarily follow. This cleaning, which will be referred to hereinbelow as the post-tungsten CMP cleaning, is essential to totally or at least substantially remove the slurry residuals and metal particles that still remain at the end of the chem-mech polishing/touch-up steps before the next processing step of the fabrication process is performed in order to avoid any potential source of contamination.
As a matter of fact, modern chem-mech polishing tools in-situ incorporate a so-called scrubbing device to eliminate this wafer particulate contamination. Scrubbing the wafer with Polyvinyl Alcohol (PVA) brushes and de-ionized (DI) water is the state-of-the-art technique to date to perform post-tungsten CMP cleaning. This simplified clean-up technique has been improved so far by the addition of chemicals (e.g. a surfactant, an acid, an alkaline . . . ) to the de-ionized water in order to obtain a substantial removal of the slurry residuals and metal particles. A suitable choice of the cleaning chemistry is very important, because it controls the electrokinetic interactions in the wafer surface-brush-particle system and the particle detachment mechanism from both the wafer surface and the brush. An acid solution, e.g. hydrofluoric (HF) acid diluted in DI water (2:100 ratio) with a pH of about 2 or an alkaline solution, e.g. ammonium hydroxide (NH4OH) diluted in DI water (1:100 ratio) with a pH of about 10 are typical examples of cleaning chemistries used to date.
PVA brushes are compatible with such cleaning chemistries in the 2 to 12 pH range.
FIG. 1 summarizes the standard three-step tungsten chem-mech polishing process as described above, which bears numeral 10. As apparent in FIG. 1, the wafer is first chem-mech polished, typically with an alumina base slurry (box 11), then, the "touch-up" is performed with a different slurry (box 12), next, the wafer is cleaned by scrubbing with an alkaline or acid solution and finally dried (box 13). At the end of the CMP process, polished wafers are usually referred to as "W-CMP wafers".
Unfortunately, this standard CMP process described by reference to FIG. 1 that is performed for planarization purposes, is not totally satisfactory. It still remains an acute particulate contamination problem caused by slurry residuals and particles of the metals used in the conductive interconnect scheme that are left at the wafer surface at the end of the CMP process as a whole. For instance, when the conventional ferric nitrate/alumina based slurry is used, ferric nitrate, alumina and metallic particles (or aggregates) are commonly found at the wafer surface at this stage of the fabrication process at an unacceptable amount.
Recently, this acute problem of particulate contamination was addressed in an article of T. L. Myers, M. A. Fury and W. C. Krusell entitled: "Post-tungsten CMP cleaning: issues and solutions" published in Solid State Technology, October 1995". Still using conventional cleaning solutions similar to those mentioned above, these authors have thoroughly studied the role of the zeta potential which was considered as the determining parameter of any attempt in the particulate contamination reduction of W-CMP wafers. The following excerpt is illustrative in that regard:
"Zeta potential offers a viable explanation for the mechanisms observed in tungsten cleaning. The enhanced cleaning ability of alkaline (high pH) solutions arises from the strong relationship between the pH and zeta potentials of these solutions and other materials present in the cleaning environment. FIG. 2 illustrates zeta potential versus pH for alumina, silica, PVA, and tungsten. When the zeta potentials between the brushes, slurry, and wafer surface are all the same sign (either positive or negative), as in a typical oxide CMP process, it is easier for the particles to be removed from the wafer. Tungsten cleaning, which follows a low pH, alumina based CMP process, is more difficult because the zeta potential differences between materials have changed sign; the alumina particles of the slurry exist in a positive regime, in contrast with the rest of the wafer environment. This zeta potential dependence emphasizes the need to know what type of CMP process was used before the wafer is cleaned. Two different processes can produce very different levels of cleaning ability. For example, a tungsten CMP process followed by an oxide "touch-up" step produces a wafer surface similar to the standard oxide process. In contrast, a tungsten process followed by only a short buffing step, the wafer would require cleaning with the high pH chemical and an increased cleaning time in order to achieve better particle control."
According to the teachings of this article, it is recommended to use alkaline with a high pH range rather than an acid cleaning mixture to ensure that all the components involved in the cleaning (wafer, brushes, . . . ) are at the same zeta potential. But, because the "touch-up" step is performed after the CMP polishing step, the wafer still remains highly contaminated so that the cleaning step which just follows does not have the expected efficiency.
As a matter of fact, it thus appears that both the prior art cleaning chemistries and the step sequence depicted in FIG. 1 do not have the desired contamination effect reduction that is required to date to meet the drastic requirements of advanced semiconductor wafer fabrication specifications. Moreover, said prior art chemistries have other inconveniences. An acid solution not only attacks the slurry residuals but also attacks the liner causing there by damages to the wafer surface. On the other hand, cleaning with an ammonium hydroxide solution is known to degrade the surface of the mirror-polished wafers by performing an undesired pit etching thereof. However, ammonium hydroxide based solutions are by far the most commonly cleaning chemistries used to date to remove the slurry residuals and metal particles on the W-CMP wafer surface.
Finally, it is to be noted that in the standard CMP process depicted in FIG. 1, the cleaning step and the "touch-up" step employ totally different cleaning chemistries which is a further inconvenience.